Methods and apparatus for the formation of heterogeneous ion-exchange membranes

ABSTRACT

The present invention provides methods and apparatus for the formation of heterogeneous ion-exchange membranes by prescribed in-line compounding and extrusion of a polymeric binder and heat sensitive ion-exchange resin. The ion-exchange resin is incorporated, at a late process stage, into the melted matrix polymer at relatively low temperature and residence time prior to transfer to a die head for extrusion. In the presently preferred embodiment, the in-line compounding apparatus comprises a twin-screw compounding extruder, for effecting late stage kneading and mixing of ion-exchange resin and optional additives to the polymer melt, prior to compression to transfer the blended polymer melt to a die head for extrusion. Accordingly, the final properties of the resultant heterogeneous ion-exchange membrane are enhanced as the blended polymer melt material is not exposed to excessive heat and shear history. Resultant heterogeneous ion-exchange membranes and apparatus for treatment of fluid streams utilizing such membranes are also provided.

BACKGROUND OF THE INVENTION

The present invention provides unique heterogeneous ion-exchangemembranes, methods and apparatus for producing such membranes, andion-removing apparatus utilizing such membranes.

Purification of fluids such as water, beverages, chemicals and wastestreams can be accomplished in a variety of different systems for aplurality of different end results. For ultrapure and drinking waterpurposes, purification may require the removal of substantial amounts ofions contained within brackish or salt water, may require the removal ofturbidity and large particles, or may require the destruction of livingorganisms. Such purification may also require removal of substantialamounts of ions from reverse osmosis permeate and DI permeate.

For removal of ions, several basic systems have found commercialacceptance: ion-exchange, reverse osmosis, electrodialysis andelectrodeionization.

In general, established methods for deionizing fluids include:distillation, ion exchange, electrodialysis, and reverse osmosis.Distillation separates water from contaminants by transferring waterinto vapor phase, leaving most contaminants behind. Ion-exchange removesions from solutions by exchange of salts for hydrogen and hydroxideions. Electrodialysis uses membranes that remove salts by ion transferunder the influence of a direct electrical current. Reverse osmosis usesmembranes that are permeable to water but not to solutes, with waterbeing purified as it is driven by pressure through the membranes.Electrodeionization (EDI) processes combine the use of ion-exchangeresins and membranes to deionize water. EDI equipment is capable ofefficient deionization of a wide range of feeds from bulk salt removalto polishing of reverse osmosis product water.

Typically, in electrodeionization, a number of flat sheets ofalternating cation and anion exchange membranes are placed between twoelectrodes with mixed bed of ion-exchange resins alternately addedbetween the membranes.

The compartments containing the resin beads are generally referred to asthe dilute compartments. The adjacent compartments into which ions aretransferred for disposal are referred to as the concentratecompartments. The concentrate compartments usually are much thinner thanthe dilute compartments, and serve to collect the concentrated ionsbeing transferred from the dilute compartments. The concentratecompartment may or may not contain additional ion-exchange resin.

When fluid flow is fed through the system, and electrical potential(voltage) is applied, ions begin to migrate towards the electrodes; theanions to the anode and the cations to the cathode.

In the dilute compartments, ions are able to cross into the neighboringconcentrate compartments only when they encounter the ‘right’ membrane;that is, when anions encounter anionic membranes and cations encountercationic membranes.

In the concentrate compartment, ions continue their migration to theelectrodes, but now they encounter the ‘opposite’ membranes; that is,anions encounter cationic membranes while cations encounter anionicmembranes. These membranes block their motion, trapping them in theconcentrate compartment where they are rinsed out.

The net result of the EDI process is that water is continuouslydeionized in the dilute compartments, with the unwanted ions exitingfrom the concentrate compartments.

U.S. Pat. No. 4,465,573 issued to Harry O'Hare for Method and Apparatusfor Purification of Water describes such devices and the advent ofelectrodeionization that continues to gain commercial acceptance amongvarious end users.

A critical element of such purification devices is the membrane thatselectively allows diffusion and adsorption of ions while excludingcertain other ions and non-ionized solutes and solvents. These membraneshave commonly been referred to as ion-exchange membranes and are used ina wide variety of devices for fractionation, transport depletion andelectroregeneration, purification for treatment of water, food,beverages, chemicals and waste streams. Such membranes are also used inelectrochemical devices and electrophoresis as well as analyticalequipment and for treatment applications.

Commercially available ion-exchange membranes are generally classifiedas two types: homogeneous membranes and heterogeneous membranes. Ahomogeneous membrane is one in which the entire volume of the membrane(excluding any support material that may be used to improve strength) ismade from the reactive polymer. Heterogeneous membranes, on the otherhand, are formed of a composite containing an ion-exchange resin toimpart electrochemical properties and a binder to impart physicalstrength and integrity.

The ion-exchange resin particles serve as a path for ion transferserving as an increased conductivity bridge between the membranes topromote ion movement. Under conditions of reduced liquid salinity, highvoltage and low flow, the resins also convert to the H+ and OH− formsdue to the splitting of water into its ions in a thin layer at thesurface of the resin particles or membranes. This further improves theattainable quality of water. During electrodeionization, the ionconcentration within the resin particles is maintained relativelyconstant and the migration of ions from the resin particles into theconcentration compartments is substantially balanced by the migrationsof the same, or similar ions from the water being purified into theresin particles.

Such membranes should be resistant to elevated temperatures, result in alow pressure loss, and result in low internal and external leaks. Thelow pressure loss reduces pumping requirements and also allows themembranes to be spaced more closely to each other, thereby reducingpower consumption caused by the electrical resistance of the waterstreams. For selective ion electrodialysis, selective ion-exchangeresins can be used as the resin component of the inventive membrane. Fortransport depletion electrodialysis, mixed anion and cation resins, oramphoteric resins can be used in place of the resin component of one ofthe anion or cation membranes. For transport of large, multivalent orslow diffusing ions, low cross-linked ion exchange resins can be used inthe membrane.

Typically, the starting ion-exchange resin bead has the physicalcharacteristic in appearance as a translucent, spherical bead with aneffective size of from about 0.25 to about 0.75 mm. Chemical stabilityof ion-exchange resins is dependent among other factors on operatingtemperatures that generally should not exceed 285 degrees F. for cationexchange resin and 195 degrees F. for anion exchange resin. The resinbeads are generally produced by a process incorporating cross-linkedpolystyrene with an active functional group such as sulfonic acid(cation) or quaternary ammonium functional groups (anion).

The foregoing membranes are useful in apparatus of reverse osmosis (RO),electrodialysis (ED) and electrodialysis reversal (EDR) processes. Suchmembranes are particularly useful for electrodeionization andelectrodeionization reversal applications, where the reduction inleakage and pressure loss is important, along with the advantage ofbeing able to readily bond the membranes within the device. Chemicalresistance is particularly important because elements and ions such ashydrogen, hydroxide, hydronium ions, oxygen and chlorine may be producedin situ, in electrodeionization devices. Furthermore, the smoothness ofthe membrane simplifies automation of resin filling and removal ofbackwashing of the resin between membranes. Finally, the elimination ofadhesives reduces the level of extractables, a significant advantagewhen electrodeionization apparatus is used in ultrapure waterproduction.

A wide variety of such membranes are known to the art. In this respect,such membranes are described for instance, in U.S. Pat. Nos. 3,627,703;4,167,551; 3,876,565; 4,294,933; 5,089,187; 5,346,924; 5,683,634;5,746,916; 5,814,197; 5,833,896; and 5,395,570.

U.S. Pat. No. 5,346,924 to Giuffrida discloses a heterogeneousion-exchange membrane using a binder comprising a linear low densitypolyethylene (LLDPE) or a high molecular weight high densitypolyethylene (HMWHDPE) and methods for making the same. The membrane isfabricated from granules or pellets of ion-exchange resin and eitherLLDPE or HMWHDPE binder that are used as a raw material in athermoplastic extrusion process, a heat pressing process, or another,similar process employing pressure and heat to create a dry compositesheet of constant width and thickness or having other controlled, formeddimensions. Membrane sheets formed by such processes are thenconditioned and activated using a water treatment.

Conventionally, heterogeneous ion-exchange membranes are fabricated byproviding granulated or powdered polymer binder to a mixer and heatinguntil the material becomes molten. Ion-exchange resins are then added inpowder form and the resulting composition is then mixed to evenlydistribute the ion-exchange resins throughout the melt. The molten castmixture may then be cast or alternatively sent to an extruder.

Where the molten mixture is cast to strand form, the strand is generallycooled and then pelletized. The pellets are thereafter fed to anextruder or other polymer processing device that combines heat andpressure. Melting and film formation is generally carried out atrelatively high temperatures, e.g., 300-350 degree F. range.

Kojima, et al., in U.S. Pat. No. 3,627,703 discloses a polypropyleneresin composite which comprises a polypropylene resin matrix that isboth microscopically foamed and molecularly oriented in three dimensionsand ion-exchanging material dispersed therein. In one embodiment, thecomposite is produced by a process which comprises subjecting aprecursor composite comprising a solid polypropylene matrix and anion-exchange material of greater swellability to a chemical treatmentcomprising an acid and an alkali treatment. In one embodied form, thepolypropylene resin and ion-exchange material by kneading at atemperature above the melting point of the polyproplylene resin.Subsequent to kneading at high temperature, the mixture is thereafterformed or molded and thereafter chemically treated.

While recognizing the virtues of polypropylene as a binder, Kojima, etal., in U.S. Pat. No. 3,627,703 discloses a fabrication process forion-exchange membrane exposing the resinous material to multiplemeltings and temperature cycles.

Accordingly those skilled in the art have recognized a significant needfor an efficient process for the fabrication of heterogeneousion-exchange membranes that accurately controls processing parameters topreserve the active ion sites and other desired characteristics of theincorporated resinous material while at the same time, providing anheterogeneous ion-exchange membrane with the structural integrityrequired for demanding environment such as electrodeionization. Thepresent invention fulfills these needs.

SUMMARY OF THE INVENTION

The present invention provides unique methods and apparatus for theformation of heterogeneous ion-exchange membranes by prescribed in-linecompounding and extrusion of a polymeric binder and heat sensitiveion-exchange resin. The ion-exchange resin is incorporated, at a lateprocess stage, into the melted matrix polymer at relatively lowtemperature and residence time prior to transfer to a die head forextrusion. In the presently preferred embodiment, the in-linecompounding apparatus comprises a twin screw compounding extruder, foreffecting late stage kneading and mixing of ion-exchange resin andoptional additives to the polymer melt, prior to compression to transferthe blended polymer melt to a die head for extrusion. Accordingly, thefinal properties of the resultant heterogeneous ion-exchange membraneare enhanced as the blended polymer melt material is not exposed toexcessive heat and shear history. Resultant heterogeneous ion-exchangemembranes and apparatus for treatment of fluid streams utilizing suchmembranes is also provided.

In a presently preferred embodiment, the inventive method comprises:

a) feeding a supply of polymer binder to an in-line compoundingextruder, having means for melting, kneading and transferring thepolymer binder to a die head for extrusion; said extruder further havingmeans for feeding additives to the melted polymer binder at a prescribedprocessing stage;

b) maintaining the polymer binder within said extruder at a temperaturerange of between about the softening point of said polymer binder andthe melting point of said polymer binder to form a melted matrixpolymer;

c) kneading the melted matrix polymer to form a homogeneous matrix;

d) subsequently adding and mixing powdered ion exchange resin to themelted matrix polymer derived from step c) to form a homogenous blendedmatrix within said extruder during a relatively limited residence time;and

e) transporting the blended, melted polymer matrix derived from step d)to a die head for extrusion to form a heterogeneous ion-exchangemembrane.

Following extrusion, the unique membranes are preferably washed in adeionized water bath at a temperature of about 180 degrees F. for atleast two hours until expansion is effected.

In a presently preferred embodiment, the inventive apparatus comprises atwin-screw compounding extruder, said extruder having a first feed zone,a second melting zone, a third zone for kneading melt homogeneity, meansfor feeding selective additives to the polymer melt down stream of saidthird zone, a fourth zone for effecting further kneading and mixing ofadditives to the preferred polymer melt, a fifth zone for mixingextrusion agents within the blended polymer melt and a sixth compressionzone to transfer the blended polymer melt to a die head for extrusion.

An optional computer processing unit can continually monitor and correctthe balance of the extrusion system to effect the method for theformation of heterogeneous membranes in accordance with the presentinvention. The control software preferably utilizes an algorithm programto analyze the prescribed inputs from key points in the extrusionsystem, makes numerical calculations, and effects any necessarycorrections to the extruder screw RPM, temperature range, residence timeand feed rate.

The preferred polymer matrix comprises about 20% to about 80% by weightof the preferred polymer melt to be extruded from the die head. Thepreferred polymer binder for the matrix is metallocene propylene polymerbased on single-site catalysis that produces polymers with very narrowmolecular weight distribution (MWD), uniform composition distributions(CD) and narrow tacticity distributions (TD). The preferred polymer hasa relatively low melting point within a range of from about 125 to about130 degrees C. The narrow molecular weight distribution of metallocenepropylene polymer provides a unique rheology that allows for extrusionof thin films. Moreover, melt flow rate (MFR) can be targeted preciselyin the reactor reducing processing variability downstream andeliminating the need for post-reactor controlled rheology (CR). Themolecular weight capability has an MFR range of between about 0.01 toabout 5,000. Typical molecular weight distribution of the preferredpolymer is about 2.0. The narrow molecular weight distribution andnarrow tacticity distribution coupled with the elimination of CRprocessing, substantially reduces low molecular weight molecules thussignificantly reducing extractables.

The ion-exchange resin to be dispersed in the polymer binder, may be anyion-exchange material which is anionic, cationic, amphoteric, or anotherionic type may be used. Preferably, ion-exchange resins which are stableat the melting point range of the preferred polypropylene resins areused for preparing the blended polymer matrix.

Accordingly, the heterogeneous ion-exchange membranes in accordance withthe present invention are particular useful for fabrication ofelectrodeionization modules. The inventive methods provide an efficientand cost effective process for formation of such membranes that exhibitenhanced properties because the resinous ion-exchange material is notexposed to excessive heat and shear history.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic block diagram illustrating multiple zones for theembodied in-line compounding apparatus in accordance with the presentlypreferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides unique methods and apparatus for theformation of heterogeneous ion-exchange membranes by prescribed in-linecompounding and extrusion of a polymeric binder and heat sensitiveion-exchange resin. The ion-exchange resin is incorporated, at a lateprocess stage, into the melted matrix polymer at relatively lowtemperature and short residence time prior to transfer to a sheet diehead for extrusion.

Accordingly, the final properties of the resultant heterogeneousion-exchange membrane are enhanced as the blended polymer melt materialis not exposed to excessive heat and shear history.

Typically, organic molecules are composed of a skeleton of carbon atoms,sheathed in hydrogens, with groupings composed of other atoms attachedto that skeleton. These attached groups are referred to as functionalgroups, since they are always the sites of chemical reactivity orfunction.

In this respect, it is generally recognized the energies involved inholding two atoms together in a covalent bond are as follows:

1. Kinetic energy (motion) and heat (essentially molecular motion)

2. Potential energy arising from

a) Electrical forces (attraction of unlike, repulsion of like charges)

At higher temperature, the energy of random molecular motion increasesand can often exceed certain bond energies and thus cause covalent bondbreaking.

In a presently preferred embodiment, the inventive method comprises:

a) feeding a supply of polypropylene binder to a compounding extruder,having means for melting, kneading and transferring the polymer binderto a sheet die head for extrusion; said extruder further having meansfor feeding and blending active additives in-line to the melted polymerbinder at a prescribed point along the extruder;

b) maintaining the polymer binder within said extruder at a temperaturerange of between about the softening point of said polymer binder andthe melting point of said polymer binder to form a melted matrixpolymer;

c) kneading the melted matrix polymer to form a homogeneous matrix;

d) adding and mixing powdered ion-exchange resin, to the melted matrixpolymer derived from step c) to form a homogenous blended melt withinsaid extruder during relatively limited residence time; and

e) compressing and conveying the blended melt derived from step d)directly to a sheet die head for extrusion to form a heterogeneousion-exchange membrane.

Following extrusion, the unique membranes are preferably washed in adeionized water bath at a temperature of about 180 degrees F. for atleast two hours until expansion and full hydration are effected.

It is critical in accordance with the present invention that the ionexchange resins be added to the polymer matrix after the matrix hasundergone melting and initial kneading. This late stage processing ofthe ion exchange resins minimizes the occurrence of covalent bonddestruction of active functional groups.

The ion-exchange material to be dispersed in the composite, may be anyion-exchanging material which is anionic, cationic, amphoteric, oranother ionic type may be used.

Representative particulate resins which can be utilized in accordancewith this invention include gel and macroporous ion-exchange resins suchas sulfonated polystyrene-divinylbenzene and aminatedpolystyrene-divinylbenzene either in pure form or in mixtures (Type I,Type II or Type III) such as those available under the trademark DOWEXfrom the Dow Chemical Company; and; chromatography resins; bifunctionalion-exchange resins such as ion retardation resins (Biord AG11A8) orion-exchange resins containing both sulfonate and quaternary aminefunctionality, sulfonated phenolic resin, polystyrene phosphoric acid oriminodiacetic acid resins, aminated acrylic or methacrylic resins, epoxypolyamine resins, aminoethyl cellulose or the like.

The polymer matrix comprises from about 20% to about 80% by weight ofthe polymer melt to be extruded from the die head. The preferred polymerfor the matrix is metallocene polypropylene polymer based on single-sitecatalysis that produces preferred polymers with very narrow molecularweight distribution (MWD), uniform composition distributions (CD) andnarrow tacticity distributions (TD). The preferred polymer has a meltingpoint within a range of from about 125 to about 130 degrees C. Thenarrow molecular weight distribution of metallocene polypropylenepolymer provide a unique rheology that allows for extrusion of thinfilms. Moreover, melt flow rate (MFR) can be targeted precisely in thereactor reducing processing variability downstream and eliminating theneed for post-reactor controlled rheology (CR). The molecular weightcapability has an MFR range of between about 0.01 to about 5,000.Typical molecular weight distribution of the preferred polymer is about2.0. The narrow molecular weight distribution and narrow tacticitydistribution coupled with the elimination of CR processing,substantially reduces low molecular weight molecules thus significantlyreducing extractables in the resultant membrane.

One preferred polymer for the matrix is a polypropylene polymer sold byEXXON under the brandname ACHIEVE™. The singlesitedness of the EXXPOLcatalyst results in a narrow tacticity distribution (TD) and alsoresults in a narrow composition distribution (CD) in random copolymers(RCP). The single sitedness gives rise to polymer performance advantagein the general area of cleanliness.

In the presently preferred embodiment, the in-line compounding apparatuscomprises a twin-screw compounding extruder for effecting late stagekneading and mixing of ion-exchange resin and optional additives to thepolymer melt, prior to compression to transfer the blended polymer meltto a sheet die head for extrusion.

The twin-screw extruder can either be co-rotating or counter-rotating.Process parameters may be manually or automatically controlled includingscrew rpm, feed rate, temperatures along the barrel and die, and vacuumlevel for devolatilization. Readouts preferably include melt pressure,melt temperature, and motor amperage. The motor inputs energy into thescrews and the rotating screws impart shear and energy into the processto mix the components, devolatilize, and pump as required.

The feeder system to the twin-screw extruder should ensure attainablepressure stability in the front end of the extruder to ensuredimensional stability of the resultant membrane. Preferably gravimetricfeeders are used for direct extrusion from the twin-screw extruder forimproved compositional accuracy inherent with their use.

The means for mixing the additive(s) to the matrix may be dispersive ordistributive. Preferably, narrower mixing elements are used in theinventive system as they are more distributive with high melt divisionrates with minimal elongational and planar shear. Distributive mixingelements allow many melt divisions without extensional shear.

The pressure gradient in the twin-screw extruder will be determined bythe selection of screws. Flighted elements can be placed strategicallyso that the screw channels are not filled and there will be a zeropressure underneath downstream vent/feed barrel sections, whichfacilitates downstream sequential feeding and prevents vent flooding.

Preferably the powdered ion-exchange material which is sized to smallerthan 100 mesh, or preferably sized to smaller than 32 mesh, is added tothe melted matrix polymer through means of a side stuffer to enter asecond kneading and mixing zone. The second mixing zone is provided witha side feed entry port that introduces the powdered additive to themelted matrix polymer, i.e., homogeneous polypropylene polymer. Thesecond kneading and mixing zone is maintained at a temperature above themelting point of the polypropylene with atmospheric venting. Thereafter,the blended melted polymer matrix and ion-exchange material is fed to athird kneading and mixing zone where extrusion agents may be added.Typically, such extrusion agents comprise glycerine and the like tofacilitate further processing transfer and extrusion through the diehead. The third kneading and mixing zone is preferably maintained undervacuum conditions for degassing and the melted mixture is thereaftertransferred through a compressional section to the die head.

The unique heterogeneous polypropylene ion-exchange membranes inaccordance with the present invention were thus formed by a twin-screwcompounding extruder. In this respect, the twin-screw extrudercontinuously mixes, devolatilizes and processes the metallocenepolypropylene binder through prescribed compounding with the resinousmaterial by relatively small shear and extentional forces. Accordingly,the traditional pelletizing step and remelting is bypassed avoidingexcessive heat and shear history.

The following is an illustrative example of the inventive method andapparatus.

FIG. 1 illustrates a schematic block diagram of a presently preferredembodiment of the inventive in-line compounding apparatus in accordancewith the present invention. As shown in FIG. 1, the supply of polymerbinder is fed, for instance, by a gravity feed device 10 to the firstzone 12 within the extrusion system. A second zone 14 effects melting ofthe polymer binder within the extruder at a temperature range of betweenabout the softening point of the polymer binder and the melting point ofthe polymer binder to form a melted matrix polymer. In a third zone 16,the melted matrix polymer is kneaded to form a homogeneous matrix. In afourth zone 18, optional additives may be supplied to the polymermatrix, for instance, conventional extrusion agents such as glycerine toenhance the malleability of the homogenous matrix. By separate gravityfeed device 20, powdered ion-exchange resin is added to the meltedmatrix polymer in the fifth zone 22 and the blended matrix is furthermixed and kneaded before degassing in the sixth zone 24. In a seventhzone 26, the blended, melted polymer matrix is compressed and fed to asheet die head 28 for extrusion to form a heterogeneous ion-exchangemembrane.

A heterogeneous polypropylene ion-exchange membrane was produced byfeeding a supply of metallocene propylene polymer to a twin-screwcompounding extruder, said extruder having a first feed zone, a secondmelting zone, a third zone for kneading melt homogeneity, a feed entryport disposed down stream of the third zone, a fourth zone for effectingfurther kneading and mixing of additives to the preferred polymer melt,a fifth zone for mixing extrusion agents within the blended polymer melta sixth zone for degassing and a seventh compression zone to transferthe blended polymer melt to a sheet die head for extrusion. The binderwas maintained within a polymer melt section of the extruder at atemperature below about 130 degrees C. to melt said binder and to kneadto form a homogeneous melt. The kneaded melted matrix polymer wasthereafter transported to an intermediate mixing zone and powderedion-exchange resin was added to the melted matrix polymer withsubsequent kneading and mixing the melted matrix polymer with theion-exchange material at a temperature below about 130 degrees C. atatmospheric pressure. The blended, melted polymer matrix was thentransported to a compression zone of the extruder. The blended, meltedpolymer matrix was thereafter transported from said compression zone toa sheet die head for extrusion to form a membrane having an extrudedthickness of approximately 0.001 inches to about 0.050 inches.

Preferably, the resultant membrane has a thickness within a range ofbetween about 0.005 and 0.025 inches. For EDI applications, theresultant member has a thickness within a range of 0.008 to 0.012inches.

Typically, the residence time of the ion-exchange material in theextrusion system will be under two minutes and preferably less thanthirty seconds.

Accordingly, the present invention provides an apparatus for theformation of a heterogeneous ion-exchange membrane comprising in asingle machine: a twin-screw compounding extruder, said extruder havinga first feed zone, a second melting zone, a third zone for kneading melthomogeneity, means for feeding selective additives to the polymer meltdownstream of said third zone, a fourth zone for effecting the kneadingand mixing of additives to the preferred polymer melt, a fifth zone formixing extrusion agents within the blended polymer melt, which may beplaced anywhere after said zone three, a sixth compression zone fordegassing the blended polymer melt, and a seventh compression zone totransfer the blended polymer melt to an attached sheet die head; inaddition, an adjustable sheet die head for extruding thin melted sheetmembrane, a roll stack for forming, cooling and calendaring themembrane, and a membrane take-up device; wherein the residence time ofthe ion-exchange material is kept to a minimum while at elevatedtemperatures, ideally less than two minutes, and a preferably to lessthan one minute.

We claim:
 1. A method for the formation of an improved heterogeneousion-exchange membrane comprising: a) feeding a supply of a polymerbinder to an in-line compounding extruder, having means for melting,kneading and transferring the polymer binder to a sheet die head forsheet extrusion; said extruder further having means for feeding andblending additives in-line to the melted polymer binder at a prescribedprocessing stage; b) maintaining the polymer binder within said extruderat a temperature range of between about the softening point of saidpolymer binder and the melting point of said polymer binder to form amelted matrix polymer; c) kneading the melted polymer binder to form ahomogeneous matrix; d) subsequently adding and mixing a powderedion-exchange resin, to the melted matrix polymer derived from step c) toform a heterogeneous blended melt within said extruder, wherein saidion-exchange resin has a residence time within said extruder of lessthan about two minutes; and e) transporting the blended melted polymermatrix derived from step d) directly into a sheet die head forcontinuous sheet extrusion.
 2. The method for the formation of animproved heterogeneous ion-exchange membrane as defined in claim 1, andfurther including the step of washing the sheet extrusion in a deionizedwater bath at a temperature of from about 180 degrees F. for at leasttwo hours until expansion is effected.
 3. The method for the formationof a heterogeneous ion-exchange membrane as defined in claim 1, whereinsaid powdered ion-exchange resin is added to the melted matrix polymerin a range of between about 20% to about 80% by weight.
 4. The methodfor the formation of a heterogeneous ion-exchange membrane as defined inclaim 1, wherein the polymer binder is polypropylene polymer.
 5. Themethod for the formation of a heterogeneous ion-exchange membrane asdefined in claim 1, wherein the powdered ion-exchange resin has anaverage size of 200 mesh.
 6. The method for the formation of aheterogeneous ion-exchange membrane as defined in claim 1, whereinpolymer binder is metallocene polypropylene polymer having a narrowmolecular weight distribution and having a melting point below about 130degrees C.
 7. The method for the formation of a heterogeneousion-exchange membrane as defined in claim 1, wherein the powderedion-exchange resin has an average size of 325 mesh.
 8. The method forthe formation of a heterogeneous ion-exchange membrane as defined inclaim 1, wherein the powdered ion-exchange resin is of Type I.
 9. Themethod for the formation of a heterogeneous ion-exchange membrane asdefined in claim 1, wherein the powdered ion-exchange resin is of TypeII.
 10. The method for the formation of a heterogeneous ion-exchangemembrane as defined in claim 1, wherein the powdered ion-exchange resinis of Type III.
 11. The method for the formation of a heterogeneousion-exchange membrane as defined in claim 1, wherein the powderedion-exchange resin is anionic.
 12. The method for the formation of aheterogeneous ion-exchange membrane as defined in claim 1, wherein thepowdered ion-exchange resin is cationic.
 13. The method for theformation of a heterogeneous ion-exchange membrane as defined in claim1, wherein the powdered ion-exchange resin is amphoteric.
 14. The methodfor the formation of a heterogeneous ion-exchange membrane as defined inclaim 1, wherein the powdered ion-exchange resin is a mixture ofion-exchange materials selected from the group consisting of: Type I,Type II, Type III, anionic, cationic, amphoteric and mixtures thereof.